Generally, gear mechanisms, such as a trapezoidal thread worm gear mechanism, or a rack and pinion gear mechanism are used as the mechanism to convert rotary motion of an electric motor to an axial linear motion in an electric actuator used in various types of driving sections. These motion converting mechanisms involve sliding contact portions and thus power loss. Accordingly, they are obliged to increase the size of the electric motors and power consumption. Accordingly, the ball screw mechanisms have been widely adopted as more efficient actuators.
In general, in electric actuators with a ball screw mechanism where a nut is contained in a housing of the electric actuator, it is necessary to provide a groove to prevent rotation of the nut. Alternatively, the inner circumference of the housing includes a profiled configuration (non cylindrical configuration) in order to prevent rotation of the nut. However, since it is difficult to machine an elongate housing with the groove or profiled configuration, the whole length of the housing should be limited. Thus, it is impossible to obtain a long stroke of the actuator. The groove or profiled configuration may be formed by a method other than machining such as by a molding method using dies. However, the molding method is complicated and leads to an increase of the manufacturing cost. In addition, the nut should have a projection to engage the groove if the groove is formed on the inner circumference of the housing. Thus, this also increases the manufacturing cost of the nut. Furthermore, both the inner circumference of the housing and the nut has complicated configurations and thus increase the configuration of the actuator.
To solve these problems, an electric actuator is known and shown in FIG. 6. The electric actuator has a housing 51, a threaded screw shaft 56, a nut 51, a plurality of balls 57 and a guide member 63. The threaded screw shaft 56 is formed with an external thread 56a. The nut 53 is formed with an internal thread 53a on its inner circumference to engage the threaded screw shaft 56 so as to be axially moved. The plurality of balls 57 are rollably arranged along a rolling passage formed between the opposite threads 56a and 53a. The guide member 63 includes a guiding portion 63a, extending along the inner circumference of the housing 51, and an engaging portion 63b, engaging the housing 51.
The housing 51 has a main body 51a with a substantially hollow configuration. A cup-shaped cover member 51b is secured to the main body 51 to close the end of the main body 51a. A cylindrical output shaft 52, forming a driven member, and a nut 53, connected to the output shaft 52, are contained in the main body 51a. They have a cylindrical inner circumference. The left-hand end of the output shaft 52 projects from the main body 51a of the housing 51. The right-hand end of the output shaft 52 is formed with a blind bore 52a. The outer circumference of the output shaft 52 is slidably supported relative to the main body 51a by a bushing 54. A sealing member 55, arranged adjacent to the bushing 54, seals a gap between the output shaft 52 and the main body 51a to prevent entry of dust from the outside.
The threaded screw shaft 56 passes through the nut 53 and extends freely movably into the blind bore 52a of the output shaft 52. The nut 53 is arranged around the threaded screw shaft 56. The nut 53 has an axially extending groove 53b at the bottom of its outer circumference. Also, the nut 53 includes a tube-type ball circulating member. The ball screw mechanism is formed by the nut 53, the threaded screw shaft 56, balls 57 and the guide member 63.
The threaded screw shaft 56 is rotationally supported by a ball bearing 59 arranged at the end of the main body 51a, via a bearing spacer 58. An outer ring of the ball bearing 59 is secured on the bearing spacer 58 by a securing member 60. An inner ring of the ball bearing 59 is secured on the threaded screw shaft 56 by a securing ring 61. Accordingly, the threaded screw shaft 56 is supported so that it is only able to rotate and not able to move axially. A gear 62 is integrally connected to the right-end of the threaded screw shaft 56 via a serration connection.
The guide member 63 is press-formed from a metal sheet. The guide member 63 has the straight guiding portion 63a and the engaging portion 63b. The straight guiding portion 63a is adapted to engage the groove 53b of the nut 53. The engaging portion 63b is bent perpendicularly to the guiding portion 63a. In addition, the right-hand end of the main body 51a of the housing 51 is formed with a recessed groove 51c to receive the engaging portion 63b. The engaging portion 63b has substantially the same width as that of the recessed groove 51c that it engages (see FIG. 7). The surface of the guiding portion 63a is coated by manganese phosphate salt or zinc phosphate.
When an operator turns the switch “ON”, a motor (not shown) is actuated and rotational power is transmitted to the nut 53. Accordingly, the threaded screw shaft 56 is rotated. The nut 53 is smoothly guided only in the axial direction by the guide member 63. The guide member 63 exhibits the rotation-preventing function. The rotational motion of the threaded screw shaft 56 can be efficiently converted to linear motion of the nut 53. Thus, the output shaft 52 to which the nut 53 is connected can be moved in the axial direction.
The electric actuator can be assembled first by inserting the guide member 63 into the main body 51a of the housing 51. The bushing 54 is assembled from the right-hand end of the main body 51a. The bent engaging portion 63b then engages the recessed portion 51c of the main body 51a. Such a structure enables easy mounting the guide member 63 to the main body 51a without using any special tool. The nut 53 and the output shaft 52 can be inserted into the main body 51a by engaging the guiding portion 63a via the groove 53b. The other parts, such as the threaded screw shaft 56, ball bearing 59 etc. can be orderly assembled.
The tip end of the guiding portion 63a engages a notch 54a of the bushing 54. Thus, both ends of the guide member 63 are firmly secured to the main body 51a. Accordingly, torsion of the guide member 63 can be effectively prevented even though the guiding portion 63a is long. Thus, the rotation-preventing function of the guide member 63 can be effectively exhibited (see, Japanese Laid-open Patent Publication No. 232338/2008).
According to the prior art electric actuator, the rotation-preventing function can be achieved by engaging the guiding portion 63a with the groove 53b formed on the outer circumference of the nut 53 and by engaging the tip end of the guiding portion 63a with the notch 54a of the bushing 54. This increases the number of parts, assembling steps and the degree of complication. Also, it requires surface properties, such as hardness, surface roughness etc., to be sufficient to stand the sliding motion between the output shaft 52, nut 53 and guiding portion 63a. Accordingly, this increases the manufacturing cost of the electric actuator.